Gut epithelial apoptosis is increased in both animal models and human autopsy studies of sepsis. This appears to be physiologically significant since overexpression of either the anti-apoptotic protein Bcl-2 or the cytoprotective peptide EGF in the intestinal epithelium inhibits apoptosis and improves survival in murine models of sepsis. The mechanisms through which prevention of gut apoptosis improves survival are unclear. This proposal examines the relationship between gut apoptosis and other determinants of gut integrity in sepsis. Under homeostatic conditions, cells proliferate in the crypt, migrate up the villus and then die by apoptosis in a cycle that occurs over 3-5 days. In addition to increased gut apoptosis, sepsis leads to markedly decreased gut epithelial proliferation. This proposal will test whether proliferation is dependent on apoptosis in sepsis by measuring proliferation in animals with increased or decreased sepsis-induced gut epithelial apoptosis and testing mechanisms that potentially control gut proliferation. In light of significantly different signaling pathways between the proliferative crypt and differentiated villus, the importance of the crypt/villus microenvironment will also be tested by studying the relationship between proliferation and apoptosis in transgenic animals that have ectopic villus proliferation. Another important determinant of gut integrity is gut barrier function. Sepsis induces gut barrier failure with intestinal hyperpermeability which can result in harmful antigens and bacteria escaping the lumen and perpetuating the systemic inflammatory response syndrome. This proposal will determine whether changes in the tight junction with resultant changes in permeability are dependent upon gut apoptosis. This will be done by measuring permeability and expression of tight junction mediators in animals with increased or decreased sepsis- induced gut epithelial apoptosis. Further, mechanisms that potentially control sepsis- induced intestinal hyperpermeability will be assayed. Finally, we have shown that while sepsis induces gut apoptosis, this is further augmented in the absence of lymphocytes, demonstrating that lymphocytes play an anti-apoptotic role on the gut epithelium in sepsis. The mechanisms underlying lymphocyte control of sepsis-induced gut apoptosis are unknown, and this proposal seeks to clarify them in highly translational models of sepsis. PUBLIC HEALTH RELEVANCE: Gut apoptosis is markedly increased in sepsis, a disease that kills 210,000 people annually in the United States. Increased gut apoptosis appears to be functionally significant since preventing gut apoptosis improves survival in animal studies of sepsis; however, the mechanisms responsible for this are unknown. Understanding how sepsis- induced gut apoptosis alters overall intestinal integrity may therefore have significant therapeutic implications in a disease that is the most common killer in intensive care units.